Cocoa Core Competencies

Property list

A property list is a representation of a hierarchy of objects that can be stored in the file system and reconstituted later. Property lists give applications a lightweight and portable way to store small amounts of data. They are hierarchies of data made from specific types of objects—they are, in effect, an object graph. Property lists are easy to create programmatically and are even easier to serialize into a representation that is persistent. Applications can later read the static representation back into memory and recreate the original hierarchy of objects. Both Cocoa Foundation and Core Foundation have APIs related to property list serialization and deserialization.

Property List Types and Objects

Property lists consist only of certain types of data: dictionaries, arrays, strings, numbers (integer and float), dates, binary data, and Boolean values. Dictionaries and arrays are special types because they are collections; they can contain one or multiple data types, including other dictionaries and arrays. This hierarchical nesting of objects creates a graph of objects. The abstract data types have corresponding Foundation classes, Core Foundation types, and XML elements for collection objects and value objects, as shown in the following list:

Collectively, instances of these classes are known as property list objects. For example, an NSMutableDictionary object is a property list object, as are NSNumber object, an NSString object, and so on. For a property list to be valid, all objects in the object graph must be property list objects.

Best Practices for Property Lists

You can write property lists out in both XML and binary formats. The binary format is much more compact than the XML version and thus more efficient. It is recommended for most situations. However, you can manually edit an XML property list if you ever need to. Property list files have the filename extension of plist.

You should not use property lists to store large, complex graphs of objects, especially when the objects have variable mutability settings. And you cannot use property lists to store objects that are not supported by the architecture, such as model objects. For these cases, use archiving instead. Although property lists can include NSData objects, it’s best to not use data objects in property lists to hold large amounts of binary data.

Property List Serialization

To serialize and deserialize property lists, call the appropriate class methods of the NSPropertyListSerialization class or, if using Core Foundation, the facilities related to the CFPropertyListRef opaque type. In Cocoa, the serialized output is in the form of an NSData object. You can therefore use the methods of that class (for example, writeToFile:atomically:) to write that data to the file system and use the appropriate NSData class factory memory to read it back into memory. Then, when you deserialize it, you can specify mutability options for the property list.